In the photolithography technology, an exposure tool for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has hitherto been widely utilized. With the trend toward a higher degree of integration and a higher function of an integrated circuit, the refinement of the integrated circuit is advancing. The exposure tool is hence required to form a circuit pattern image with high resolution on a wafer surface at a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source is further advancing from conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer layer (wavelength: 193 nm) is employed. Also, in order to cope with a next-generation integrated circuit whose circuit line width will become 65 nm or less, an immersion lithography technique and a double exposure technique, each using an ArF excimer laser, are employed. However, it is considered that even these techniques would be able to cover only the generation with a line width of up to 22 nm.
Under the foregoing technical trends, a lithography technique using, as an exposure light source, light having a wavelength of 13 nm to represent EUV light is considered to be applicable over generation of 22 nm and thereafter, and is attracting attention. The principle of image formation of the EUV lithography is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The EUVL optical member used in an exposure tool for EUVL includes a photomask and a mirror. In the case of the photomask, the EUVL optical member is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate and (3) an absorber layer formed on the reflective multilayer. In the case of the mirror, the EUVL optical member is basically configured with (1) a substrate and (2) a reflective multilayer formed on the substrate. For the reflective multilayer, forming an Mo/Si reflective multilayer in which an Mo layer and an Si layer are alternately laminated is investigated; and for the absorber layer, Ta and Cr are investigated as a film-forming raw material. For the substrate, a material having a low coefficient of linear thermal expansion (CTE) is required so as not to generate a strain even under irradiation with EUV light. For the material having a low CTE, a silica glass containing TiO2 (hereinafter, it may be referred to as “TiO2—SiO2 glass” in the specification) is known as an extremely low thermal expansion material having a CTE lower than that of a quartz glass. Also, since the CTE can be controlled by the TiO2 concentration in glass, a zero-expansion glass whose CTE is close to 0 can be obtained. Accordingly, the TiO2—SiO2 glass is investigated to be used as a substrate of an EUVL optical member.
In carrying out EUVL, the temperature in the exposure tool for EUVL is strictly controlled to be 22±3° C. for the purpose of preventing change in dimension of EUVL optical members such as a photomask and a mirror in the exposure tool caused by temperature changes thereof. In this state, the temperature of a mounting surface of the EUVL optical member in the exposure tool for EUVL reaches 22±3° C. However, it is suggested that, among the EUVL optical members, the temperature at the incidence surface side for EUV light is increased by the irradiation of high energy EUV light, and the temperature becomes higher than the temperature (22±3° C.) at the mounting surface side.
For this reason, it is preferred that the substrate of EUVL optical member has a wide temperature region at which CTE is substantially zero. However, the conventional TiO2—SiO2 glass has a narrow temperature region at which the CTE is substantially zero, and the glass was not sufficient to use in the substrate of EUVL optical member. In the present specification, the term “the CTE is substantially zero” means that the CTE is 0±50 ppb/° C., and preferably 0±25 ppb/° C.
Patent Document 1 discloses that a low expansion glass member having an average CTE change at the use temperature is 1 ppb/° C./° C. or less is obtained by preparing a TiO2—SiO2 glass having temperature at which CTE is 0 ppb/° C. (Cross-Over Temperature: COT) of 30° C. or higher.